The invention relates generally to metrology systems, and more particularly to a system for writing or calibrating an encoder scale at an end-use installation.
U.S. Pat. No. 4,950,890, which is hereby incorporated herein by reference in its entirety, discloses a method and apparatus for writing position markings onto a blank encoder scale at an end-use installation, where the encoder scale and an associated read/write head will be used to monitor the position of a moving part. The '890 patent teaches that an accurate position transducer such an interferometer is used to directly measure the position of a selected portion of the moving part and to command the encoder read/write head to write position marks on the blank encoder such that they correspond to the actual position of the selected portion of moving part. The '890 patent teaches that imperfect bearings systems and other errors sources generally cause discrepancies between conventional encoder position measurements and the actual positions of moving parts that they are intended to monitor. By writing an encoder scale as outlined above, it is intended that the encoder scale incorporate scale distortions that automatically compensate for such sources of errors.
However, the method of the '890 patent has various disadvantages in various applications. Misalignment of the interferometer or other reference relative to the motion axis may cause the scale to be written with an improper scale factor. In many applications, accurately aligning an interferometer or other accurate position reference with the actual motion axis, as well as with a desired portion of the moving part, may be uneconomical, may require skills beyond those of a typical end user, and/or may be impractical due to space constraints, or the like. In addition, depending on the internal configuration of the encoder read/write head and its nominal alignment relative to the scale, the effective writing path or writing angle between the readhead writing element and the scale may generally vary from the effective reading path or reading angle between the readhead reading element and the scale. In such a case, dynamic misalignments between the readhead and scale, such as a changing gap or angle due a scale that is not flat, or the like, may introduce encoder-related errors that are not compensated for by the method of the '890 patent. An improved method for writing or calibrating an encoder scale at an end-use installation that overcomes these and other disadvantages, either individually or in combination, would be desirable.